This invention relates to servo boosters and, more particularly, to improvements in a mechanism for connecting a valve body and a piston plate of a pneumatic servo booster which constitute the power piston of the servo booster.
In a conventional vacuum servo booster which is well-known as one kind of pneumatic servo booster, a power piston is constituted by a valve body disposed at the central portion of the servo booster and a piston plate which is disposed around the periphery of the valve body and which is adapted to receive a differential pressure produced across a diaphragm. In such case, the piston plate and the valve body are usually connected by bolts which also act to clamp the inner periphery of the diaphragm, and the connecting mechanism comprises a plurality of threaded holes formed in the valve body, holes in the piston plate and the bolts inserted from the side of the piston plate. However, an air passage is provided in the valve body, and thus it is necessary to arrange the threaded holes radially outside of the air passage which inevitably increases the outer diameter of the valve body. Further, in attaining the desired tightening force in such a screw-thread connection, the threaded holes are required to have a depth greater than a predetermined amount thus increasing the thickness of the valve body. Further, there are problems in the strength with respect to the tightening stress produced around the threaded holes and with respect to the thermal stress when the valve body is formed of a synthetic resin material. Further, the bolts project from the front surface of the piston plate thereby occupying a space which could otherwise be utilized to hold other parts of the servo booster.
In order to solve the problems aforementioned there has been proposed a vacuum booster wherein an aperture and a head portion are provided respectively at the center portion of the piston plate and at one end (the inner end) portion of the valve body and which are engageable with each other at a predetermined angular position, whereby the piston plate is secured to the valve body by being rotated through a predetermined angle in the aperture with the inner circumference of the diaphragm being clamped between the plate and, a neck portion which is formed adjacent to the head portion of the valve body.
However, since the relative rotation of the piston plate and the valve body is prevented only by the interference of the diaphragm in the direction of the thickness thereof, there is the shortcoming that the valve body will rotate relative to the piston plate due to vibrations or to the decrease in the interference due to the aging of the diaphragm during a long period of usage thereby permitting the valve body to become engaged from the piston plate.